|So you've tried both USB and PS/2 keyboards? Try a different PS/2 keyboard.|
Could be a bad PS/2 socket. Or you've got the keyboard plugged into the PS/2 mouse port.
I don't know about your particular motherboard but older ones often had a pin-out for a 'keyboard lock' which could be connected to a keyswitch on the front panel. I doubt motherboards have that anymore but you can check. If it's there and those pins are jumpered, your keyboard won't work.
Also older boards had a keyboard fuse, which if blown, prevented your keyboard from working. It was located on the motherboard by the keyboard port. It was a surface mount fuse that looked like a resistor and you couldn't tell it was blown just by looking at it. Again, I don't know if motherboards still have those.